Relatively planar dielectric surfaces over closely spaced features are desired in many stages of integrated circuit manufacturing. Such surfaces minimize problems that might be caused by electrical discontinuities created by reentrant angles in deposited metals, limited depth of focus of steppers, etc. Unfortunately, most dielectrics are conformal in nature and are frequently deposited over nonplanar surfaces and their surfaces are similar to the underlying surfaces, i.e., they are nonplanar. The problems associated with nonplanar surfaces become more severe as device dimensions continue to decrease. The term planar surface is used to mean local planarization; i.e., the surface may be considered planar over closely spaced features, although it is not necessarily planar over the entire range of features.
A typical situation leading to a nonplanar surface involves forming an electrical contact to a narrow region located between two features such as runners. This region may be thought of as a trench, i.e., its longitudinal dimension is much greater than is its transverse dimension and the vertical dimension is relatively large. The region may be a source/drain region and the runners, which will likely have sidewalls, will now be less than a micron apart. Because of the small spacing between features and their sidewalls, the space for the contact is considerably less than a micron in the shortest direction, although it may be more than a micron in the longitudinal direction.
The small contact size makes it difficult to print the contact window accurately, and self-aligned contacts have been developed to minimize alignment problems. The runners are, for example, polysilicon covered by silicon nitride. A thin silicon oxide layer is deposited over the runners and a portion of the layer is then selectively removed after being defined lithographically to define the contact area using the nitride as an etch stop. This method is not completely satisfactory because the typical etch ratio for oxide and nitride is relatively small, for example, 2:1, and the etch may go into the polysilicon. Additionally, a nonplanar surface is left for further processing.
Even if the contact area is suitably patterned as described, problems may arise when the contact material, for example, polysilicon, is deposited and patterned. As will be readily appreciated, the polysilicon must be removed everywhere in the trench between the runners except in the contact area. It is difficult to remove the polysilicon completely from the trench. Additionally, there may be some material, i.e., resist or residue, in the trench. This material acts as an etch stop and makes accurate patterning impossible.
Other approaches to solving problems associated with nonplanar surfaces also exist. For example, a planarizing resist etchback may also be used. This suffers from several drawbacks. For example, the lack of selectivity of oxide with respect to nitride makes the opening of high aspect ratio windows difficult because nitride is likely to be removed from the tops of the runners.
It is desired to make the electrical contact with a process that can print a window having a dimension greater than the spacing between the feature.